In mammals, neurons destined for the adult cerebral cortex are generated only after the period when two transient neuronal populations are born. These populations are called the subplate and the marginal zone cells, and they differentiate into many chemically and morphologically distinct cell types before undergoing preprogrammed cell death. Subplate neurons send axons into the thalamus, midbrain and contralateral hemisphere early in fetal life, supporting the idea that they function as pioneer neurons. They are also the first cortical cell types to receive functional thalamo-cortical synapses. Most subplate cells eventually die, but only after the adult pattern of cortico-cortical and thalamo-cortical connections become established. Together, these and other observations indicate that subplate neurons have a number of important functions in establishing cortical circuits. Little, if anything is known about molecular mechanisms underlying these functions. An immunological approach to search for subplate specific molecules has been used to produce a new monoclonal antibody which stains neurons in the cortical subplate (Naegele et. al.'89). This new antibody selectively stains the cortical subplate when neurons in this region are undergoing preprogrammed cell death. The goal of the proposed pilot studies is to further characterize this new mAb antibody, called Subplate-1, and the cells that express this antigen. In these pilot studies,Subplate-1 will be used in combination with 3H-thymidine birth-dating autoradiography to determine whether the earliest generated neurons of the subplate are stained. The morphology and neurotransmitter phenotypes of the stained neurons will be determined by using Subplate-1 in combination with intracellular staining and double-label immunocytochemistry. Finally, biochemical approaches will be used to characterize the antigen itself. These studies will require neonatal cats and ferrets principally because nearly all previous work on the times of subplate cell genesis, morphology, connectivity and neurochemistry have been studied in these species. However, preliminary results indicate that mAb Subplate-1 also stains fetal monkey and human subplate, so it is likely that any new findings can be related to the development of the human cerebral cortex. Eventually this information may further our understanding of the role of the subplate in normal cortical development and in congenital defects of this brain region.